The present invention relates generally to the field of telecommunications and, in particular, to the suppression of stimulated Brillouin scattering in optical transmissions.
Telecommunications systems transmit signals between user equipment, e.g., telephones, radios, and computers, over a network. Conventional telecommunications systems include, but are not limited to the public switched telephone network (PSTN), the Internet, wireless networks, and cable television networks. These networks typically include transmission media such as coaxial cable, copper wires, optical fibers, and wireless links, e.g., radio and satellite communications.
In optical transmission systems, one of the optical transmitter types that feed relatively high-powered optical signals into long fiber lengths is a configuration designed around a Mach-Zehnder modulator. The modulator is generally fed from a relatively high power laser. The laser operates in the cool white mode and provides the xe2x80x9clight sourcexe2x80x9d that has its intensity or amplitude modulated in the Mach-Zehnder device. The optical modulation is accomplished by feeding the radio frequency (RF) modulating signal to the appropriate amplitude modulation (AM) port of the modulator. In this way RF amplitude modulation is converted into optical amplitude modulation.
A detrimental characteristic of feeding high optical powered signals into long lengths of fiber is that noise and distortion are generated when the stimulated Brillouin scattering (SBS) launch power threshold of the fiber is exceeded. Stimulated Brillouin scattering (SBS).is a nonlinear optical effect that poses a significant restriction to the amount of narrow-line width optical power that can be launched into a long length of single-mode optical fiber. For a given length of single-mode fiber with a given attenuation coefficient at the chosen optical wavelength, there is an optical-linewidth-dependent threshold power below which SBS does not occur. In order to launch high optical signal powers, for example, in the 1550 nm wavelength region for transmission of broadband signals over long fiber distances, SBS needs to be suppressed. SBS creates excessive noise in the received signal and causes distortion.
FIG. 5a illustrates that when the power of a wavelength of interest, xcex1, exceeds the SBS launch power threshold (Psbs) another, undesired wavelength, xcexn, is generated. The undesired wavelength xcexn has a very noisy and somewhat distorted signal content. Both of these wavelengths propagate down the fiber. At the end of the fiber this degraded and unwanted wavelength impinges on the optical receiver along with the wavelength of interest. The noise from the undesired wavelength will be demodulated along with the normal signal from the wavelength of interest. The distortion and primarily the noise from the undesired wavelength severely degrade the desired signal.
These detrimental effects are experienced if the power of any one of many individual optical wavelength signals launched into a long fiber exceeds the SBS threshold. It is not the total aggregate optical power that is critical in determining the SBS launch power threshold and its associated signal degradation. The SBS effects are also related to the length of the fiber. The longer the fiber, the lower the SBS threshold and the more severe the problem.
The total net optical power of many wavelengths that impinge on a receiver determines the quality of the received signal. However, the SBS threshold is based on only the individual power of the strongest wavelength. One conventional approach to assuring compliance with the SBS threshold is to distribute the optical power of one wavelength over many wavelengths. This is conventionally accomplished by phase modulating an amplitude modulated optical signal emanating from the output of a typical optical modulator. This phase modulation can be achieved in an optical modulator, such as a Mach-Zehder optical modulator, with a built-in and separately fed phase modulation port. This can also be achieved by using a separate optical phase modulator in series with an AM.
Driving the phase modulation port with a sufficiently large signal produces optical sidebands that follow standard Bessel function characteristics as illustrated by the graph of FIG. 5b. Using the correct drive level lowers the power of the main wavelength, xcex1, below the critical SBS launch power threshold (Psbs) to a new power level P1. The new wavelength xcex1xe2x80x2 and the other wavelengths xcex2xe2x80x2 to xcex5xe2x80x2 that are produced are each lower in power than the original signal, xcex1, shown in FIG. 5a. The total power of all of the sidebands theoretically equals the power of the original wavelength xcex1 before the phase modulation was applied. All of the wavelengths will contain the desired RF AM modulation content, and all are normally xe2x80x9cdetectedxe2x80x9d by the photo-detector of the optical receiver to effectively produce the desired RF output signal from the optical receiver. The RF frequency at the phase port should be high enough to prevent harmonics and mixed RF components from contaminating the main RF signal content and typically can be a microwave frequency in the 1-3 Ghz range.
This technique for reducing the desired wavelength below the SBS threshold only provides a small margin of protection. With the increased need for power in transmissions, this technique provides limited improvements in transmission capabilities in optical systems. In optical transmission systems and especially in systems having long unrepeated fiber spans, it is important to launch as high an optical power into the transmission fiber as possible. Unfortunately, the amount of launch power usable at a particular wavelength is limited by SBS. The SBS degrades the optical signals and increases bit error rates for the data transported by the transmission system.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improvements in compensating for the effects of SBS in telecommunications systems.
The above mentioned problems with optical transmission in long fiber lengths and other problems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification.
In one embodiment, a transmitter is provided. The transmitter includes a non-linear device having an optical input adapted to receive an optical signal, an amplitude modulation input adapted to receive an amplitude modulation signal, a phase modulation input and an output. The transmitter also includes a stimulated Brillouin scattering (SBS) oscillator/driver having first and second oscillators coupled to the phase modulation input of the non-linear device and an amplifier coupled to the output of the non-linear device. The transmitter further includes a laser coupled to the optical input of the non-linear device.
In another embodiment, a transmitter is provided. The transmitter includes an optical modulator that includes an input adapted to receive a radio frequency signal, an optical signal input, and an output. The transmitter also includes a phase modulator having an optical input, a phase modulation input and an output. The output of the phase modulator is coupled to the optical input of the optical modulator. The transmitter further includes an SBS oscillator/driver coupled to the phase modulation input of the phase modulator, an amplifier coupled to the output of the optical modulator and a laser coupled to the optical input of the phase modulator.
In another embodiment, a communication system is provided. The communication system includes a transmitter having a non-linear device with an optical input, an amplitude modulation input, a phase modulation input and an output. The transmitter also includes a stimulated Brillouin scattering (SBS) oscillator/driver having first and second oscillators coupled to the phase modulation input of the non-linear device. The communication system also includes at least one receiver and at least one transmission line coupled between the transmitter and the at least one receiver.
In a further embodiment, a method of suppressing stimulated Brillouin scattering in an optical transmission is provided. The method includes receiving a radio frequency input signal, amplitude modulating the radio frequency signal and feeding the amplitude modulated radio frequency signal to an amplitude modulation input of a non-linear device. The method also includes generating an optical signal using a laser and feeding the optical signal to an optical input of the non-linear device. The method further includes frequency modulating the optical signal using a microwave radio frequency signal generated by a first oscillator, frequency modulating the microwave radio frequency signal using a second oscillator which is coupled to an input of the first oscillator and feeding the frequency modulated microwave radio frequency signal to a phase modulation port of the non-linear device.